Tomatoes are an important food source. They are consumed either raw or in a processed state; it is common to prepare sauces or soups from them.
The popularity of tomatoes may be ascribed to their sweet-sour fresh flavour, which is dependent on their sugar to acid ratio and a combination of at least four hundred volatile aroma compounds which are released or generated when the tomato fruit is cut. For example, aldehydes and alcohols having 6 carbon atoms (C6) contribute to the fresh green aroma of tomatoes; they are generated by lipid oxidation.
To prepare tomato-based products, fresh tomatoes are traditionally processed using either a cold or a hot break process, in which tomatoes are comminuted and heated to temperatures of about 65 and 85.degree. C. respectively. A concentrated tomato product is prepared by evaporation of water from these comminuted and heated tomatoes. However, many volatile aroma compounds such as C6 aldehydes and C6 alcohols are released into the atmosphere during these processes, resulting in a significant loss of the desirable green aroma of fresh tomatoes.
An alternative method for producing concentrated tomato products is to use reverse osmosis to separate solids and liquids. In the separated solids, volatile aroma compounds such as C6 aldehydes are retained in relatively high amounts. However, C6 aldehydes are detrimental to aroma quality in higher concentrations, since the aroma becomes too green and thus grassy and chemical, rather than moderately green and fresh.
Typically, fresh tomato homogenate comprises from 5.0 to 20.0 ppm C6 aldehydes and from 0.2 to 6.0 ppm C6 alcohols; concentrated tomato paste prepared using the hot break process and evaporation comprises from 0 to 0.2 ppm C6 aldehydes and from 0 to 0.1 ppm C6 alcohols; concentrated tomato paste prepared using reverse osmosis comprises from 0.1 to 20 ppm C6 aldehydes and from 0.1 to 20 ppm C6 alcohols.
In U.S. Pat. No. 5,436,022, pre-ripened whole tomatoes are blanched in hot water to inactivate surface enzymes, cooled, macerated in the presence of added alkali and alcohol to provide a slurry having a pH of 4.7-5.1, acidified and subjected to a traditional hot-break process. During the maceration step, endogenous enzymes are released and effect enzymatic reactions. The resulting tomato products are sweeter, more fruity and less sour than tomato products prepared by conventional hot break processes. This is because the activities of the enzymes responsible for the development of the desired fruity and sweet taste are utilised before the hot break process is used to inactivate them.
An article in the Journal of Food Science, vol 54, no 6, 1989, p 1607-1610 relates to the formation of n-hexanol from n-hexanal by enzyme action in soybean extracts. At a pH of 8-9, it is assumed that n-hexanal is reduced by endogenous alcohol dehydrogenase to produce n-hexanol (although no specific enzyme is shown). The presence of cofactors NADH and NADPH is stated to be necessary for this reduction to occur. The conversion of n-hexanal to n-hexanol is said to be useful in reducing the intensity of the grassy, beany, green flavours of n-hexanal.
In relation to tomatoes, "Constituents Volatils de la Tomato: Mise en evidence et Formation par Voie Enzymatique du tran-Hexene-2-ol" in the Agr. Biol. Chem., 40(12), 2349-2353, 1976 discusses the action of isolated alcohol dehydrogenase on trans-2-hexenal to produce trans-2-hexenol.